Current techniques for tracking groundwater or subsurface solutions typically involves indirect geophysical methods such as various forms of galvanic resistivity, electromagnetic conductivity, ground penetrating radar, or the drilling and logging of observation wells.
Geophysical methods can be broadly classified into two groups—passive and active. Passive methods detect variations in the earth's natural fields, such as gravitational or magnetic fields. These methods are not suited for characterizing preferential groundwater flow paths and usually cannot resolve acute subsurface changes. Although passive methods have been used with limited success in mapping groundwater flow paths, the data obtained by the method can be very difficult to interpret and is generally unreliable without other supporting data.
Active methods, on the other hand, transmit manmade signals into the earth—such as sound waves or electric currents—which are subsequently measured after passing through and being modified by subsurface materials. Active methods are better suited for many types of exploration, including groundwater characterization.
Some geophysical methods energize a large volume of the subsurface-combining and averaging the earth's response between rock, soil and water resulting in a mixed bag of responses that are often difficult to interpret and insensitive to acute changes in subsurface conditions. Over the past several years, three-dimensional (3D) seismic methods have proven to be helpful in characterizing groundwater bodies. However, a major drawback to 3D seismic methods is the cost and difficulty in performing surveys on land (i.e., over difficult to access land such as steep terrain, highly vegetated and/or populated areas). Electrical methods such as direct current (DC) resistivity and electromagnetic (EM) methods are used more commonly for groundwater characterization. These methods energize the subsurface volume as a whole and utilize non-conductive overburden to achieve good penetration and signal fidelity. Successful application of these technologies can be limited by wet clays, conductive soils, and surface water. Data acquisition is also highly affected by electrical interference and metal objects. Even with good penetration, the averaging of signals that have travelled through a large volume of subsurface material yields ambiguous results. Altogether, these methods have serious limitations as tools for groundwater characterization.
Another active technique that may be used to characterize groundwater is ground penetrating radar (GPR). This technique uses electromagnetic wave propagation and scattering to identify changes in electrical properties. This technique can be more accurate. However, GPR depth resolution is limited to only a few meters or less if clays are present and is not well-suited for coverage of large areas.
Drilling and logging of wells is another option for identifying and/or tracking subsurface water solutions. A drawback to drilling is that drilling does not reveal more than what is at the location of the drill hole. To establish linkage between holes, a tracer solution or some geophysical continuity test is often used. Geophysical techniques used to establish connectivity between holes is to place an electrode in one hole at the horizon of interest and then lower another electrode in the second hole to see if there is a response at the horizon of interest in the second hole. This technique may establish connectivity but does not provide a surface trace of the path that the water follows between the drill holes. Confidently mapping a subsurface water system, following subsurface pollution plumes, identifying all branches of a groundwater source, or recognizing all offshoots of a pollution plume can be difficult with drilling. In addition, drilling can miss a narrow stream of groundwater with a well and thus produce inconclusive or misleading results.
A method to map groundwater plumes using electrical resistance tomography (ERT) and electro kinetic system (EKS) was developed which places many electrodes on the surface and in wells and measures all combinations of resistivity between them. The water or fluids are then caused to move using electro kinetics. Subsequently, the various resistivity combinations are remeasured. This data is combined to create a tomography picture that results from the displacement of the plume.
Despite the development of the various technologies listed, and others, such methods and technologies are inadequate for hydrogeologic characterization. Also, ERT, EKS, IP, and so forth are limited in effectiveness when other electrical lines, magnetic fields, underground infrastructure (including metal), and so forth are present. Additionally, use of such technologies within densely populated areas can be difficult.